Not many know about this, but the year after King John reluctantly signed the Magna Carta, both the king and the barons had soon broken the terms of that agreement and were in a full-blown civil war. Upon their invitation, Louis the Dauphin of France (and son of King Philip II) landed a French army upon English shores, and was on the brink of conquering the last holdouts of King John's kingdom when the latter suddenly died from dysentery, and his son (the future King Henry III) assumed the throne. Given the ire of the Anglo-Norman barons were directed at King John, and not his son, they switched their loyalties, and forced Louis to withdraw his forces back to France.
What if King John had survived his illness, and along with his son was later found and then killed by joint French and Anglo-Norman forces? What would have happened then?